The present application relates to a display device, a switching circuit and a field effect transistor, and is preferentially applied to, for example, a liquid crystal display device or an organic electroluminescence (EL) display device.
A liquid crystal display device is widely used as a flat-panel display. In some of the liquid crystal display devices, a thin film transistor (TFT) as a kind of a field effect transistor is used in a switching circuit for driving pixels. As an example of a related art liquid crystal display device using the thin film transistor, an active matrix driving liquid crystal display device of IPS (In Plane Switching) mode or FFS (Fringe Field Switching) mode is known. FIG. 17 and FIG. 18 show one form of such a liquid crystal display device. Here, FIG. 17 is a sectional view of the liquid crystal display device, and FIG. 18 is a plan view of the liquid crystal display device.
As shown in FIG. 17 and FIG. 18, in this liquid crystal display device, a TFT substrate 100 and an opposite substrate 200 are provided to be opposite to each other through a liquid crystal (not shown) therebetween, and the distance between them is regulated by a spacer 300 provided on the opposite substrate 200.
In the TFT substrate 100, a gate wiring line 102 and gate electrodes 102a and 102b branching from the gate wiring line 102 are provided on a transparent glass substrate 101. A gate insulating film 103 is provided so as to cover the gate wiring line 102 and the gate electrodes 102a and 102b. The gate insulating film 103 includes two layers of a lower insulating film 103a and an upper insulating film 103b. A silicon thin film 104 having a specified shape, which becomes a channel, is provided on the gate insulating film 103 and extends over the gate electrodes 102a and 102b. An impurity is doped in the silicon thin film 104 except portions of channel parts 104a and 104b above the gate electrodes 102a and 102b. An impurity doped region on one end side of the silicon thin film 104 forms a source region 105, and an impurity doped region at the other end side forms a drain region 106. Reference numeral 104c denotes an impurity doped region. A thin film transistor T′ of a switching circuit for driving pixels, which has a structure (or a dual gate structure) in which two thin film transistors are electrically connected in series to each other, includes the gate electrodes 102a and 102b, the gate insulating film 103, the source region 105 and the drain region 106. FIG. 19 is a plan view of a portion of the thin film transistor T′. As shown in FIG. 19, the silicon thin film 104 which becomes the channel of the thin film transistor T′ has a constant width w1 in a channel length direction.
Interlayer insulating films 107 and 108 are provided so as to cover the silicon thin film 104. A contact hole 109 is provided in portions of the interlayer insulating films 107 and 108 above the source region 105. Besides, a contact hole 110 is provided in portions of the interlayer insulating films 107 and 108 above the drain region 106. A data line 111 contacts the source region 105 through the contact hole 109. Besides, a lead electrode 112 contacts the drain region 106 through the contact hole 110. Barrier metal films 113 are provided on the data line 111 and the lead electrode 112. An interlayer insulating film 114 is provided so as to cover the data line 111 and the lead electrode 112. A contact hole 114a is provided in a portion of the interlayer insulating film 114 above the lead electrode 112. The surface of the interlayer insulating film 114 is flattened except for a portion of the contact hole 114a, and a common electrode 115 is provided on the flattened surface. The common electrode 115 has an opening 115a in a portion above the lead electrode 112, which is larger than the contact hole 114a. An interlayer insulating film 116 is provided so as to cover the common electrode 115. A contact hole 116a is provided in a portion of the interlayer insulating film 116 above the lead electrode 112. A pixel electrode 117 is provided on the interlayer insulating film 116. The pixel electrode 117 is connected to the barrier metal film 113 and the lead electrode 112 through the contact hole 116a, and is connected to the drain region 106 of the thin film transistor T′ through the lead electrode 112. A holding capacitive element C′ is formed by the structure in which the interlayer insulating film 116 is sandwiched between the pixel electrode 117 and the common electrode 115. The pixel electrode 117 has a slit-shaped opening 117a. When a voltage is applied between the pixel electrode 117 and the common electrode 115, the liquid crystal (not shown) is driven by an electric field generated between the pixel electrode 117 and the common electrode 115 in the opening 117a. 
On the other hand, in the opposite substrate 200, color filters 202 and 203 are provided on a transparent glass substrate 201 in such a manner that parts thereof overlap with each other. A flattening layer 204 is provided on these color filters 202 and 203. The spacer 300 is provided on the flat surface of the flattening layer.
In the foregoing liquid crystal display device, it is necessary that the thin film transistor T′ for pixel driving has such a sufficiently high ON current characteristic that a pixel potential can be written, and has such a sufficiently suppressed OFF current characteristic that the written potential is held. However, with improvement in definition of a liquid crystal display device and advancement of requested characteristics of optical standards, the luminance of a backlight is made high, and a very intense incident light is irradiated to the thin film transistor T′ from the backlight. Thus, the light leak current of the thin film transistor T′ increases, an OFF-current characteristic is degraded, the contrast of the liquid crystal display device is reduced, and poor picture quality due to flicker and streak-like unevenness occurs. In the thin film transistor T′, the silicon thin film 104 which is sensitive to light is used as the element region, when light is irradiated to an end of the drain region 106 of the silicon thin film 104, a carrier due to light excitation is generated in the channel part, a light leak of the thin film transistor T′ occurs by that, and the picture quality of the liquid crystal display device is badly influenced.
Hitherto, in order to reduce the light leak current of the thin film transistor T′, as shown in FIG. 20, the width of the silicon thin film 104 as the channel is made uniformly narrow in the channel length direction and a width w2 is made smaller than w1.